AAAR 36th Annual Conference October 16 - October 20, 2017 Raleigh Convention Center Raleigh, North Carolina, USA
Abstract View
Limits to the Precision of the Optical Closure Technique When Quantifying Measurement Uncertainties
JAMES RADNEY, Christopher Zangmeister, National Institute of Standards and Technology
Abstract Number: 282 Working Group: Instrumentation and Methods
Abstract The reverse optical closure technique has seen increasing use in recent years as a method to determine a refractive index for an arbitrary aerosol with biomass burning aerosols receiving the most attention. The reverse optical closure method determines a refractive index from measured absorption coefficients, scattering coefficients and size distributions assuming spherical particles (electrical mobility sizing accuracy and Mie theory). Here, we calculate refractive indices using the reverse optical closure method from measured the absorption coefficients, extinction coefficients and size distributions for pure ammonium sulfate, pure nigrosin and 50:50 (m/m) and 75:25 (m/m) mixtures of ammonium sulfate and nigrosin. We treat measured particle number densities as Poisson distributions and use quantifiable uncertainties (i.e. 0, 1, or 2 σ) to assess significance. We find that calculated refractive indices depend strongly upon the chosen confidence intervals; pure nigrosin refractive indices vary between 1.66 + 0.19i (0σ) and 1.71 + 0.21i (2σ) for Dg = 116 nm to 1.76 + 0.23i (0σ) to 1.81 + 0.25i (2σ) for Dg = 131 nm. The primary difference between the 0 and 2 σ values is the number statistically significant particles counted at large mobility diameters; notably, these particles account for less than 0.25 % of the total concentration. The best agreement with refractive indices determined from mass- and mobility- selected particles is somewhere between 1 and 2 σ depending upon the type of particles measured and the underlying size distribution of the particles under investigation. These results highlight that optical closure method is not a robust way to determine refractive index and should only be used for obtaining relative values of the refractive index.